1. Field
The present invention relates to a time synchronization offset compensation apparatus and method for orthogonal frequency division multiplexing (OFDM) symbol demodulation, a receiver comprising the apparatus, and a computer readable recording medium storing program for executing the method.
2. Description of the Background Art
FIGS. 1 and 2 are block diagrams illustrating architecture of a conventional OFDM communication system.
Referring to FIG. 1, a transmitting unit 100 comprises an Audio or Video (A/V) source 110 for generating an A/V signal; an A/V encoder 120 for source coding; a transmission MOdulator/DEModulator (MODEM) 130 for channel coding and OFDM modulation to provide resistance to signal distortion in wireless space; a Radio Frequency (RF) unit 140 for frequency conversion and signal amplification for transmission to wireless space; and a transmission antenna 150 for efficient signal transmission to wireless space. Referring to FIG. 2, a receiving unit 200 comprises a reception antenna 210 for efficiently receiving an RF signal traveling through space; an RF unit 220 for performing low-noise signal amplification and frequency conversion; an Analog-to-Digital Conversion (ADC) unit 230 for converting an analog signal into a digital signal for signal processing by a reception MODEM; the reception MODEM 240 for processing a signal by OFDM demodulation and channel decoding; an A/V decoder 250 for processing an A/V signal by source decoding; and an audio/video player 260 for reproducing a decoded A/V signal.
It is the accuracy of frequency and time synchronization that the greatest influence is had on a reception performance of an OFDM receiver under the environments of fixed communication and mobile communication in which a terminal changes in position in real time.
FIG. 3 is a schematic block diagram illustrating architecture of a conventional OFDM reception MODEM comprising a frequency and time synchronization circuit.
Referring to FIG. 3, a data buffer 2401 stores a signal inputted to the reception MODEM 240, in a unit of frame. A frame time synchronization unit 2402 processes an input signal by time synchronization, using stored data. An integer-multiple frequency synchronization unit 2403 again processes an input signal by integer-multiple frequency synchronization. A symbol time synchronization unit 2404 acquires accurate time synchronization within a range of ±½ sample. A time synchronization compensator 2406 locates an accurate starting point of a frame by applying information on the acquired time synchronization to a next subsequent frame. A decimal-multiple frequency synchronization unit 2405 receives a signal from the integer-multiple frequency synchronization unit 2403. The decimal-multiple frequency synchronization unit 2405 is applied and enabled at each OFDM symbol so as to compensate a variation of input carrier frequency caused by Doppler effect occurring in the environments where a receiver moves and a variation of intermediate frequency generated due to a short term stability of a reference oscillator frequency of a transmitter or a receiver. A frequency synchronization compensator 2408 compensates, for a desired frequency offset, an input data signal inputted as divided into an I signal and a Q signal by an I & Q demodulator 2407 after compensation of a time synchronization offset in the time synchronization compensator 2406, using a decimal-multiple synchronization offset acquired at each OFDM symbol.
The above frame time synchronization and symbol time synchronization perform a function of locating a starting point of a reference signal transmitted from a transmitter, within a range of an input signal resolution of the receiver. The frame time synchronization is performed only when the receiver powers on or when a magnitude of an input signal is suddenly reduced and synchronization is missed because of a sudden change of the external environment such as deep fading or shadowing. However, in the receiver, the symbol time synchronization unit 2404 is enabled at each frame to compensate a time-dependent varying time synchronization offset caused by a reference clock frequency difference between the transmitter and the receiver after time synchronization is acquired. This results in a problem that a time for calculation processing and an amount of power consumption are greatly increased in a real-time processing communication system (e.g., a terrestrial Digital Multimedia Broadcasting (DMB) system).